Anionic clay materials comprise synthetic or natural layered mixed hydroxides exhibiting anionic exchange abilities. The crystal structure of such compounds comprises positively charged hydroxide layers intercalated with anions and water molecules. This constitutes a very broad family of minerals since they can be formed with most divalent and trivalent cations as well as most inorganic and organic anions. Pyroaurite, hydrotalcite and sjogrenite can be taken as examples of natural minerals having this structure. See H.F.W. Taylor, Mineralogical Magazine, 39, 304, pp. 377-389, 1973. A general formula for the clay materials of interest here can be proposed as follows: ##EQU1## where x is a number less than 1, A is an anion, and y is the valence of the anion.
Natural deposits of such minerals can be found but are usually mixed with other minerals and impurities which prevent any commercial utilization. However, these anionic clays can be readily prepared synthetically by various methods and raw materials such as described in U.S. Pat. Nos. 3,539,306, 3,650,704, 3,879,523, 4,560,545, 4,539,195, 4,656,156, 4,351,814 and 4,458,026. Such synthetic materials are used in the pharmaceutical industry as antacids, in the chemical industry as halogen scavengers in polyolefins, as adsorbents, and as catalysts for chemical reactions such as isomerization of olefins (U.S. Pat. No. 4,657,307), aldol condensation (U.S. Pat. No. 4,476,324), and methanol synthesis (S. Gusi et al., Preparation of Catalysts IV, p. 753, Elsevier, 1987). Hydrogenation catalysts prepared by adsorbing ruthenium ions on a hydrotalcite support are disclosed in U.S. Pat. No. 4,923,837. A catalytic process for polymerizing epoxides using hydrotalcite catalysts is described in U.S. Pat. No. 4,962,237. Pillared hydrotalcites and catalysts using pillared hydrotalcites are described in U.S. Pat. Nos. 4,774,212 and 4,843,168, respectively.
Synthesis methods which use stochiometric amounts of raw materials without any excess of salts are preferred. Such a method is described in parent application, now U.S. Pat. No. 4,970,191 (which is incorporated herein by reference) and offers many manufacturing advantages relative to other methods, commonly known as co-precipitation methods. The main advantage of such a method is that the absence of salts obviates the washing/filtering step and produces a higher purity material. This also allows one to make gel-like materials which are usually impossible to filter but are preferred for making high crush strength catalyst pellets.
The potential for forming the material into strong pellets is imperative for the use of the material as catalysts in industrial reactors. In cases where pore diffusion affects the selectivity of the reaction, it is critical to use the smallest catalyst pellets which can be used without an unnecessarily large pressure drop. Moreover, such catalyst pellets should also contain large access pores to attain good diffusion. Both of these criteria can only be satisfied with a material and a forming method which have the ability of giving good mechanical properties.
U.S. Pat. Nos. 4,476,324 and 4,458,026 claim the calcination of the filter solids as catalysts (claim 1, steps 5-6). They also propose to form the calcined powders by extrusion or compression into tablets (page 3, lines 57-62).
U.S. Pat. No. 4,400,431 uses anionic clay materials to synthesize spinels and also proposes to form the fired material into shapes by compression and sintering. The final products can be used as catalyst carriers.
U.S. Pat. No. 4,656,156 describes the forming of calcined synthetic hydrotalcites with activated alumina to obtain composites with good mechanical strength and useful as adsorbents.
It is known in the art of forming catalysts that tableting powders produces strong pellets. However, such pellets are difficult and expensive to make in sizes less than 1/8" in diameter, which are desirable for diffusion sensitive reactions. The use of a binder such as aluminum oxyhydroxide is also not suitable because the resulting alumina, obtained after calcination, can catalyze non-desirable reactions.
The aldol condensation of acetone to isophorone is known to occur in the liquid phase in the presence of homogeneous bases such as sodium or potassium hydroxide. Several processes are described in U.S. Pat. Nos. 3,337,633, 3,981,918 and 4,059,632. Most of the isophorone product today is manufactured via these processes. Disadvantages of the liquid phase process are long residence time, high pressure equipment, high capital cost and waste streams containing the used catalyst.
Heterogeneous catalysts for aldol condensation of acetone to isophorone and mesityloxide are described in U.S. Pat. Nos. 3,946,079, 4,476,324, 4,535,187 and 4,970,191. U.S. Pat. No. 4,535,187 describes a calcium on alumina catalyst and states, (page 1, lines 48-51), "the co-precipitated mixed oxide catalysts have the drawback of exhibiting poor catalyst manufacturing reproducibility and are expensive". For this reason, the calcium on alumina catalyst is preferred. However, mesityloxide is the principal product and the crude isophorone is highly colored. A treatment of the crude isophorone is necessary to obtain low color refined isophorone and is described in U.S. Pat. No. 4,434,301.
Catalysts made with anionic clay materials have been tested with microreactors or pulse reactors as described in U.S. Pat. No. 4,476,324 for the aldol condensation of acetone. The catalyst is usually in the form of a fine powder and provides good selectivities (mesityl oxide and isophorone), i.e. 85 wt %. The mole ratio of mesityloxide to isophorone is also substantially lower than the one obtained with the calcium on alumina catalyst. However, when a commercially utilizable catalyst is used, in the form of 1/4" tablets, the selectivity decreases to 77% at 23% conversion of acetone. Such selectivity is inadequate for making isophorone cheaply and for minimizing the co-production of heavy condensation products. The decrease of selectivity from about 85 to 77 wt % is caused by pore diffusion limitations.
It is, therefore, an object of this invention to provide an efficient aldol condensation catalyst for converting acetone to mesityloxide and isophorone.
It is also an object of this invention to provide an efficient catalyst which can be used in a commercial vapor phase reactor.